The present disclosure relates to an inspection device and an inspection method, and particularly relates to an inspection device and an inspection method for inspecting an object to be inspected by using a pulsed light source.
A high-intensity pulsed light source is used in some cases for inspection of a mask for EUV (Extreme Ultra Violet) lithography (which is referred to hereinafter as EUV mask), for example, in order to improve the accuracy of inspection. Further, critical illumination is used in some cases in order to ensure the luminance of illumination light. Critical illumination is a method of illumination for forming an image of a light source on the top surface of an EUV mask, and it is an optical system achieving bright illumination.
Further, when detecting image data for inspection, in some cases an area sensor operates in TDI (Time Delay Integration) mode that transfers the pixel values of a two-dimensional (X-Y direction) area sensor in synchronization with the stage in the X direction and conducts time delay integration of the obtained pixel values. Use of the TDI mode compensates for the lack of sensitivity of the sensor and achieves highly sensitive imaging of a mask pattern.
One of EUV light sources that have been put into practical use recently is an LPP (LASER Produced Plasma) EUV light source as described in Hakaru Mizoguchi et al. “Short wavelength light source for semiconductor manufacturing: Challenge from excimer laser to LPP-EUV light source” Komatsu Technical Report March 2017, Vol. 62, No. 169, P. 27. An LPP-EUV light source is a pulsed light source that applies plasma-producing laser light to a tin droplet discharged from a droplet generator and uses EUV light produced from tin that has turned into plasma.
The LPP-EUV light source has characteristics that, while the position of a light emitting point of plasma emission and the spatial distribution of luminance (which is referred to hereinafter as luminance distribution) are stable, its emission luminance varies from pulse to pulse due to various reasons such as intensity fluctuation of plasma-producing laser light and size fluctuation of droplets, as described later. Further, because its emission timing is determined by an oscillator in the LPP-EUV light source, it cannot be determined using an external trigger signal. Furthermore, the emission period of the LPP-EUV light source has a relatively large jitter σS. The size of the jitter σS is the same or greater than the transfer period of TDI.
FIG. 1 is a view illustrating the luminance distribution of illumination light in the visual field of an area sensor when an area illuminated with an LPP-EUV light source is observed using the area sensor through an inspection optical system. FIG. 1 shows the case (a) where the luminance distribution is uniform and the case (b) where the luminance distribution is concentric. The luminance distribution of illumination light in the visual field of an area sensor tends to be non-uniform as shown in FIG. 1(b) particularly when critical illumination is employed in an illumination optical system of an inspection device.
Specifically, in the case of using an LPP-EUV light source as a light source of a lithography mask inspection device, its illumination light has characteristics that the luminance distribution is not uniform in the visual field of an area sensor, the illumination luminance varies from pulse to pulse, and the pulse emission period fluctuates. Note that, however, the position of the center of gravity of the luminance distribution is fixed and does not move for each pulse.
The lithography mask inspection device compares an inspection image taken with design data or a reference image obtained by taking an image of the same pattern on a specimen and, when they do not match, determines that there is pattern defect.
For accurate and stable pattern defect inspection, an inspection mage preferably has spatially uniform intensity distribution and is illuminated with stable illumination light whose intensity does not vary over time. When the intensity of illumination light varies over position or time, unintended variation in luminance (artifact) occurs in a mask pattern image taken, which causes an error in determination of pattern defect. There is thus a need to detect the luminance of a light source in some way and correct output fluctuation in a TDI sensor.
In the case of using a pulsed light source having characteristics that, while the positional stability of a light emitting spot is high enough, the luminance distribution is not uniform in the observation visual field, the illumination luminance varies from pulse to pulse, and the pulse emission period fluctuates, such as an LPP-EUV light source, as an illumination light source in an inspection device and using a TDI sensor as a detector that acquires image data of an object to be inspected, the following two problems need to be addressed in relation to luminance correction of illumination light.
A first problem is caused by the fact that the luminance distribution of illumination light in the visual field of a TDI sensor is not uniform as shown in FIG. 1(b). Japanese Unexamined Patent Application Publication No. 2010-091552 discloses a method that places a light quantity sensor (e.g., photodiode) for correction to detect the quantity of pulsed light and measures the entire light quantity of each pulse in synchronization with the period of pulsed light and thereby corrects the output fluctuation in the TDI sensor. However, when the luminance distribution of illumination light is not uniform, the luminance of illumination light varies depending on a light receiving spot on the TDI sensor, and therefore an error occurs in luminance correction in the method of Japanese Unexamined Patent Application Publication No. 2010-091552.
When the luminance distribution of illumination light in the visual field of a TDI sensor is not uniform, the luminance distribution may be detected using a detector having high position resolution such as a second TDI sensor, for example; however, use of the second TDI sensor complicates the optical system.
A second problem is caused by the fact that the emission timing of pulsed light emitted from a light source cannot be controlled by an external signal and that the emission period contains jitter that is the same or greater than the transfer period of TDI. FIG. 2 is a view illustrating the transfer timing of a TDI sensor and the emission timing of a light source that emits pulsed light. An inspection device using a light source that emits pulsed light and a TDI sensor generally measures the moving distance of a stage in uniform motion with a object to be inspected placed thereon and controls the transfer timing of the TDI sensor and the emission timing of the pulsed light source based on a result of the measurement. At this time, as shown in (a) of FIG. 2, the emission timing (t0 or t1) of pulsed light is synchronously-controlled to have a certain time difference TD with respect to the transfer timing of the TDI sensor.
However, as shown in (b) of FIG. 2, when the emission period Ts of pulsed light emitted from the light source contains jitter σS, synchronous control for maintaining a constant time difference TD between the transfer timing of the TDI sensor and the emission timing of pulsed light cannot be carried out, which causes overlap of the transfer timing of the TDI sensor and the emission timing of pulsed light. When the overlap between the transfer timing and the emission timing occurs, charge to be accumulated in one pixel is split into two pixels. This causes an error in luminance correction.
FIG. 3 is a view illustrating the transfer timing of the TDI sensor, the operation of the TDI sensor, the timing of an emission trigger signal, and the emission intensity of a light source. As shown in FIG. 3, the TDI sensor starts transfer operation upon input of a clock pulse indicating the transfer timing to the TDI sensor. The period of a clock pulse indicating the transfer timing of the TDI sensor is a transfer period τTi. The transfer period τTi is needed for the transfer operation of the TDI sensor.
As described earlier, the emission timing of the light source is generally controlled by an external synchronizing signal so as to have a certain time difference with respect to the transfer timing of the TDI sensor. Specifically, a clock pulse indicating the emission timing (emission trigger signal) is given with a time difference τD with respect to a clock pulse indicating the transfer timing. Receiving the clock pulse indicating the emission timing, the light source generates pulsed light. The duration of pulsed light is duration τp. The value of τD is set to the following range, and therefore the transfer period τTi and the duration τp do not overlap.τTt<τD<τTi−τp 
On the other hand, when the emission timing of a light source has jitter that is the same or greater than the transfer period τTi of the TDI, and a time difference between the transfer timing and the timing of emitting illumination light including pulsed light cannot be kept constant, the transfer timing of the TDI sensor and the emission timing of pulsed light overlap in some cases. To be specific, when an input time of a clock pulse indicating the transfer timing of the TDI sensor is τt0, the transfer period τTi and the duration τp overlap if the emission timing tp of the light source falls within the following time range.τt0−τp<tp<tt0+τTt 
When the transfer timing of the TDI sensor and the emission timing of pulsed light overlap as described above, the amount of light to be accumulated in one pixel is split into two pixels. This causes an error in luminance correction. To prevent the occurrence of such an error, jitter σS of the pulse emission timing needs to be sufficiently less than the exposure time (=transfer period τTi) of one pixel in the transfer direction of the TDI sensor (σS<<τTi).
However, because the jitter σS of the same or greater than the transfer period of the TDI sensor exists at the emission timing of an LPP-EUV light source 11, the emission timing of the LPP-EUV light source 11 fluctuates over several pixels in the transfer direction of the TDI sensor (σS to τTi). In this case, the transfer timing of the TDI sensor and the emission timing of pulsed light overlap, which increases an error in luminance correction.
The present disclosure has been accomplished to solve the problem of an error in luminance correction caused by luminance distribution of illumination light and overlap of transfer timing and emission timing, and provides an inspection device and an inspection method that enable accurate luminance correction and accurate inspection of an object to be inspected even with use of a simple detector with low position resolution.